The invention relates to the field of stents and in particular to systems which can sense the expansion and apposition of a deployed stent.
An endoluminal stent is a tubular structure inserted into a collapsing, weakened, and/or occluded passageway (e.g., blood vessel or exocrine duct) where it is expanded and left as a permanent scaffold. The most common variety is the balloon-expandable endovascular stent for the coronary artery, of which an estimated 200,000 were implanted in U.S. patients in 1997.
There are two criteria that must be satisfied for a stent implantation to be considered successful. The stent must be expanded to some minimum diameter along its length, and it must be completely in contact with the vessel wall. These two issues are not equivalent, as the diameter of a diseased coronary artery typically changes dramatically over short distances and is typically not axisymmetric. A focal lesion, for example, may require the proximal and distal ends of the stent to be further expanded than the middle in order to achieve full apposition.
It is desirable to achieve a stent expansion-monitoring system as a clinical aid in assisting optimal stent emplacement, and as an alarm warning the operator of various types of failures. As previously discussed, one endpoint criterion of successful stent implantation is achievement of some minimal lumen diameter. Failure to achieve this diameter will not only result in inadequate blood perfusion (the fundamental reason for intervention), but also may result in a lower blood-vessel-to-stent surface area ratio. This has been associated with the primary failure mode of stents: the chronic hyperproliferative growth of smooth muscle cells called neointimal hyperplasia. This growth impinges on the lumen and reduces blood flow enough in approximately 30% of all stented patients to require further intervention within three months.
An overexpanded stent can similarly cause serious complications. Although aneurysms (burst vessels) are rarely caused while stenting, post-mortem histological findings indicate that deep vessel injury caused by overstretching is a common resultant of stenting. This deep injury is believed to be the primary cause of neointimal hyperplasia and stent failure. The problem is exacerbated by the potential for the most common stent designs to open unevenly, with one hemicylindrical side overexpanded and the other underexpanded, still giving a circular lumen while hyperextending one half of the artery and inflicting deep arterial injury.
One of the two criteria for successful stent deployment is full apposition of the stent against the vessel wall, since any regions of the stent that protrude into the lumen causes blood turbulence leading to acute thrombosis and arterial blockage. The desire to prevent this leads to the common practice of dilating the stent after it is initially expanded with a high-pressure balloon. This procedure intentionally overexpands the stent to ensure full apposition, but causes unnecessary injury to the vessel, leading to neointimal hyperplasia.
The focal nature of many vessel lesions compounds this problem, as the proximal and distal ends may not be apposed to the vessel wall at the time the middle of the stent has expanded sufficiently to restore patency. FIG. 1 is a simplified cross section of a partially deployed stent 100 in a vessel 102 narrowed by plaque 104. The stent has restored patency, but is not fully apposed against the vessel wall. There does not presently exist a time- and cost-efficient method or device that can determine whether the end struts of a stent are adequately apposed.